Liquid crystal display device and electronic device

ABSTRACT

To provide a liquid crystal display device which can perform image display in both modes: a reflective mode where external light is used as an illumination light source; and a transmissive mode where a backlight is used. In one pixel, a region where incident light through a liquid crystal layer is reflected to perform display (reflective region) and a region through which light from the backlight passes to perform display (transmissive region) are provided, and image display can be performed in both modes: the reflective mode where external light is used as an illumination light source; and the transmissive mode where the backlight is used as an illumination light source. In addition, two transistors connected to respective pixel electrode layers are provided in one pixel, and the two transistors are separately operated, whereby display of the reflective region and display of the transmissive region can be controlled independently.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/978,760, filed Dec. 27, 2010, now allowed, which claims the benefitof a foreign priority application filed in Japan as Serial No.2009-298456 on Dec. 28, 2009, both of which are incorporated byreference.

TECHNICAL FIELD

The present invention relates to a semiconductor device having a circuitformed using a transistor and a manufacturing method thereof. Thepresent invention relates to, for example, an electronic device on whichan electro-optical device typified by a liquid crystal display panel ismounted as a component.

BACKGROUND ART

In a liquid crystal display device, an active matrix liquid crystaldisplay device, in which pixel electrodes are provided in matrix andtransistors are used as switching elements connected to respective pixelelectrodes in order to obtain an image with high quality, has attractedattention.

An active matrix liquid crystal display device, in which transistorsformed using a metal oxide for a channel formation region are used asswitching elements connected to respective pixel electrode, has alreadybeen known (see Patent Document 1 and Patent Document 2).

It is known that an active matrix liquid crystal display device isclassified into two major types: transmissive type and reflective type.

In the transmissive liquid crystal display device, a backlight such as acold cathode fluorescent lamp or the like is used and optical modulationaction of liquid crystals is utilized to choose one between the twostates: a state in which light from the backlight passes through liquidcrystal to be output to the outside of the liquid crystal display deviceand a state in which light is not output, whereby bright and dark imagesare displayed; further, image display is performed in combination ofthese.

Since the backlight is utilized in the transmissive liquid crystaldisplay device, it is difficult to recognize display in the environmentwith strong external light, for example, outdoors.

In the reflective liquid crystal display device, the optical modulationaction of liquid crystals is utilized to choose

one between the two states: a state in which external light, that is,incident light is reflected by a pixel electrode to be output to theoutside of the device and a state in which incident light is not outputto the outside of the device, whereby bright and dark images aredisplayed; further, image display is performed in combination of these.

Compared to the transmissive liquid crystal display device, thereflective liquid crystal display device has the advantage of low powerconsumption since the backlight is not used; therefore, a demand for thereflective liquid crystal display device as a portable informationterminal has increased.

Since external light is utilized in the reflective liquid crystaldisplay device, the reflective liquid crystal display device is suitedfor image display in the environment with strong external light, forexample, outdoors. On the other hand, it is difficult to recognizedisplay when the liquid crystal display device is used in a dimenvironment, that is, in the environment with weak external light.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-96055

DISCLOSURE OF INVENTION

It is an object to provide a liquid crystal display device which canrecognize image display even when the liquid crystal display device isused in a dim environment.

It is another object to provide a liquid crystal display device whichcan perform image display in both modes: a reflective mode whereexternal light is used as an illumination light source; and atransmissive mode where a backlight is used.

In one pixel, a region where incident light through a liquid crystallayer is reflected to perform display (reflective region) and a regionthrough which light from the backlight passes to perform display(transmissive region) are provided, and image display can be performedin both modes: the reflective mode where external light is used as anillumination light source; and the transmissive mode where the backlightis used as an illumination light source. In addition, two transistorsconnected to respective pixel electrode layers are provided in onepixel, and the two transistors are separately operated, whereby thedisplay regions of the connected pixel electrode layers can becontrolled independently.

When there is external light with enough brightness, this liquid crystaldisplay device is put in the reflective mode and a still image isdisplayed, whereby power consumption can be reduced.

When external light is weak or there is no external light, the backlightis turned on in the transmissive mode, and image display can beperformed.

A sensor for detecting brightness of the surroundings of the liquidcrystal display device is preferably provided. The reflective mode, thetransmissive mode, or on/off of the backlight is preferably performed inaccordance with data obtained using the sensor, and the amount of lightis preferably adjusted in accordance with data obtained using thesensor.

For a light source of the backlight, it is preferable to use a pluralityof light-emitting diodes (LEDs) in which power consumption can befurther reduced as compared to the cold cathode fluorescent lamp andwhich can control the strength and weakness of light. The use of LEDsfor the backlight partly controls the strength and weakness of light,whereby image display with high contrast and high color visibility canbe performed.

According to one embodiment of the present invention disclosed in thisspecification, a liquid crystal display device includes: a displaypanel; a backlight portion; and an image processing circuit. The displaypanel includes a plurality of pixels each including a pair of a firstsub-pixel and a second sub-pixel, and a first driver circuit configuredto control a pixel portion including the plurality of pixels temporally.The first sub-pixel includes a first pixel electrode which has alight-transmitting property and which is connected to a first scan lineand a first signal line and which is configured to control an alignmentstate of liquid crystal, and a transistor connected to the first pixelelectrode. The second sub-pixel includes a second pixel electrode whichreflects visible light and which is connected to a second scan line anda second signal line and which is configured to control an alignmentstate of liquid crystal, and a transistor connected to the second pixelelectrode. The backlight portion includes a plurality of light-emittingelements and a second driver circuit configured to control the pluralityof light-emitting elements temporally. The image processing circuitincludes a memory circuit configured to store image signals, acomparison circuit configured to compare the image signals stored in thememory circuit and to calculate a difference. The liquid crystal displaydevice includes: a moving-image mode in which the comparison circuitdetermines that successive frame periods in which the difference isdetected is a moving image period, the image processing circuit outputsa first signal including a moving image to the first signal line of thedisplay panel, and the image processing circuit outputs a second signalin synchronization with the first signal to the backlight portion; and astill-image mode in which the comparison circuit determines thatsuccessive frame periods in which the difference is not detected is astill image period, the image processing circuit converts a still imagein the still image period into a monochrome still image, the imageprocessing circuit outputs the first signal including the monochromestill image to the second signal line of the display panel, and theimage processing circuit stops output of a signal to the backlightportion.

According to another embodiment of the present invention disclosed inthis specification, a liquid crystal display device includes: a displaypanel; a backlight portion; an image processing circuit; and aphotometric circuit. The display panel includes a plurality of pixelseach including a pair of a first sub-pixel and a second sub-pixel, and afirst driver circuit configured to control a pixel portion including theplurality of pixels temporally. The first sub-pixel includes a firstpixel electrode which has a light-transmitting property and which isconnected to a first scan line and a first signal line and which isconfigured to control an alignment state of liquid crystal, and atransistor connected to the first pixel electrode. The second sub-pixelincludes a second pixel electrode which reflects visible light and whichis connected to a second scan line and a second signal line and which isconfigured to control an alignment state of liquid crystal, and atransistor connected to the second pixel electrode. The backlightportion includes a plurality of light-emitting elements and a seconddriver circuit configured to control the plurality of light-emittingelements temporally. The image processing circuit includes a memorycircuit configured to store image signals, a comparison circuitconfigured to compare the image signals stored in the memory circuit andto calculate a difference. The liquid crystal display device includes: amoving-image mode in which the comparison circuit determines thatsuccessive frame periods in which the difference is detected is a movingimage period, the image processing circuit outputs a first signalincluding a moving image to the first signal line of the display panel,and the image processing circuit outputs a second signal insynchronization with the first signal to the backlight portion; and astill-image display mode in which the comparison circuit determines thatsuccessive frame periods in which the difference is not detected is astill image period, the image processing circuit converts a still imagein the still image period into a monochrome still image, the imageprocessing circuit outputs the first signal including the monochromestill image to the second signal line of the display panel, and theimage processing circuit stops output of a signal to the backlightportion. The photometric circuit detects external light, so that thebacklight is adjusted in accordance with the detected external lightwhen the still-image mode and the moving-image mode are switched.

According to another embodiment of the present invention disclosed inthis specification, a liquid crystal display device includes: a displaypanel; a backlight portion; and an image processing circuit. The displaypanel includes a plurality of pixels each including a pair of a firstsub-pixel and a second sub-pixel, and a first driver circuit configuredto control a pixel portion including the plurality of pixels temporally.The first sub-pixel includes a first pixel electrode which has alight-transmitting property and which is connected to a first scan lineand a first signal line and which is configured to control an alignmentstate of liquid crystal, and a transistor which includes an oxidesemiconductor layer and which is connected to the first pixel electrode.The second sub-pixel includes a second pixel electrode which reflectsvisible light and which is connected to a second scan line and a secondsignal line and which is configured to control an alignment state ofliquid crystal, and a transistor which includes an oxide semiconductorlayer and which is connected to the second pixel electrode. Thebacklight portion includes a plurality of light-emitting elements and asecond driver circuit configured to control the plurality oflight-emitting elements temporally. The image processing circuitincludes a memory circuit configured to store image signals, acomparison circuit configured to compare the image signals stored in thememory circuit and to calculate a difference. The liquid crystal displaydevice includes: a moving-image mode in which the comparison circuitdetermines that successive frame periods in which the difference isdetected is a moving image period, the image processing circuit outputsa first signal including a moving image to the first signal line of thedisplay panel, and the image processing circuit outputs a second signalin synchronization with the first signal to the backlight portion; and astill-image mode in which the comparison circuit determines thatsuccessive frame periods in which the difference is not detected is astill image period, the image processing circuit converts a still imagein the still image period into a monochrome still image, the imageprocessing circuit outputs the first signal including the monochromestill image to the second signal line of the display panel, and theimage processing circuit stops output of a signal to the backlightportion.

With the above structure, at least one of the above problems can beresolved.

According to one object of the present invention, a plurality ofstructures is provided in one pixel and a reflective electrode isprovided on side surfaces of the plurality of structures and a pixelelectrode having a transparent electrode is provided for upper portionsof the plurality of structures.

An electronic device can be provided in which the liquid crystal displaydevice disclosed in this specification and a solar battery. The solarbattery and the display panel are opened and closed freely, and electricpower from the solar battery is supplied to the display panel, thebacklight portion, or the image processing circuit.

In this specification, a semiconductor device means all types of deviceswhich can function by utilizing semiconductor characteristics, and anelectro-optical device, a semiconductor circuit, and an electronicdevice are all semiconductor devices.

A liquid crystal display device in which image display can be performedin accordance with an environment of various brightness levels ofexternal light can be provided. Further, low power consumption can berealized in displaying of a still image.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating one embodiment of a liquidcrystal display device.

FIG. 2 is a diagram illustrating one embodiment of a liquid crystaldisplay device.

FIGS. 3A to 3C are diagrams illustrating one embodiment of a drivingmethod of a liquid crystal display device.

FIGS. 4A and 4B are diagrams illustrating one embodiment of a drivingmethod of a liquid crystal display device.

FIGS. 5A and 5B are diagrams illustrating one embodiment of a drivingmethod of a liquid crystal display device.

FIG. 6 is a view illustrating one embodiment of a liquid crystal displaydevice.

FIGS. 7A and 7B are view illustrating one embodiment of a liquid crystaldisplay device.

FIG. 8 is a view illustrating one embodiment of a liquid crystal displaydevice.

FIG. 9 is a view illustrating one embodiment of a liquid crystal displaydevice.

FIG. 10 is a view illustrating one embodiment of a liquid crystaldisplay device.

FIGS. 11A to 11D are views each illustrating one embodiment of atransistor applicable to a liquid crystal display device.

FIGS. 12A to 12E are views illustrating one embodiment of amanufacturing method of a transistor applicable to a liquid crystaldisplay device.

FIGS. 13A and 13B are diagrams illustrating one embodiment of anelectronic device.

FIG. 14 is a view illustrating one embodiment of a liquid crystaldisplay device.

FIGS. 15A to 15E are views illustrating one embodiment of a liquidcrystal display device.

FIG. 16 is a view illustrating one embodiment of a liquid crystaldisplay device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways without departing from the spiritand the scope of the present invention. Therefore, the present inventionis not construed as being limited to description of the embodiments.

Embodiment 1

In this embodiment, a liquid crystal display device including astill-image mode and a moving-image mode will be described withreference to FIG. 1. Note that in this specification, a mode performedin such a way that a display device determines image signals input tothe display device as a still image is described as a still-image mode,and a mode performed in such a way that the display device determinesthe image signals input to the display device as a moving image isdescribed as a moving-image mode.

A liquid crystal display device 100 of this embodiment includes an A/Dconverter circuit 102, an image processing circuit 110, a display panel120, and a backlight portion 130 (see FIG. 1).

The image processing circuit 110 includes a memory circuit 111, acomparison circuit 112, a selection circuit 115, a display controlcircuit 113, and a field sequential signal generator circuit 114.

The display panel 120 includes a driver circuit 121 and a pixel portion122. The pixel portion 122 includes a pixel 123. The pixel 123 includesa first sub-pixel 123 a connected to a first scan line and a firstsignal line and a second sub-pixel 123 b connected to a second scan lineand a second signal line. The sub-pixel 123 a and the sub-pixel 123 bare paired, and a plurality of the pairs is arranged in matrix as thepixel 123 in the pixel portion 122.

The sub-pixel 123 a includes a first transistor, a pixel electrodeconnected to the first transistor, and a capacitor. A liquid crystallayer is sandwiched between the pixel electrode and a counter electrodefacing the pixel electrode to form a liquid crystal element. The pixelelectrode has a light-transmitting property. Note that in thisspecification, an electrode having a light-transmitting property,through which visible light passes, is also referred to as alight-transmitting electrode or a transparent electrode.

The sub-pixel 123 b includes a second transistor, a pixel electrodeconnected to the second transistor, and a capacitor. A liquid crystallayer is sandwiched between the pixel electrode and a counter electrodefacing the pixel electrode to form a liquid crystal element. The pixelelectrode reflects incident light through the liquid crystal layer.

An example of liquid crystal elements is an element which controlstransmission and non-transmission of light by optical modulation actionof liquid crystals. The element can include a pair of electrodes and aliquid crystal layer. The optical modulation action of liquid crystalsis controlled by an electric field applied to the liquid crystals (thatis, a vertical electric field). Note that specifically, the followingcan be used for a liquid crystal element, for example: a nematic liquidcrystal, a cholesteric liquid crystal, a smectic liquid crystal, adiscotic liquid crystal, a thermotropic liquid crystal, a lyotropicliquid crystal, a low-molecular liquid crystal, a high-molecular liquidcrystal, polymer dispersed liquid crystal (PDLC), a ferroelectric liquidcrystal, an anti-ferroelectric liquid crystal, a main-chain liquidcrystal, a side-chain high-molecular liquid crystal, a banana-shapedliquid crystal, and the like. In addition, the following can be used asa diving method of a liquid crystal: a TN (twisted nematic) mode, an STN(super twisted nematic) mode, an OCB (optically compensatedbirefringence) mode, an ECB (electrically controlled birefringence)mode, an FLC (ferroelectric liquid crystal) mode, an AFLC(anti-ferroelectric liquid crystal) mode, a PDLC (polymer dispersedliquid crystal) mode, a PNLC (polymer network liquid crystal) mode, aguest-host mode, and the like.

The backlight portion 130 includes a backlight control circuit 131 and abacklight 132. The backlight 132 includes a light-emitting element 133.

In this embodiment, the backlight 132 includes a plurality oflight-emitting elements 133 which emit light of different colors. As acombination of light of different colors, for example, light-emittingelements which emit light of three colors (red (R), green (G), and blue(B)) can be used. Three primary colors of R, G, and B are used, wherebya full-color image can be displayed.

Another light-emitting element which emits a color exhibited by making aplurality of light-emitting elements selected from the light-emittingelements of R, Q and B emit light at the same time (for example, yellow(Y) exhibited by R and G, cyan (C) exhibited by G and B, magenta (M)exhibited by B and R, or the like) may be provided in addition to thelight-emitting elements of R, G, and B.

A light-emitting element which emits light of a color other than thethree primary colors may be added, so that color reproductioncharacteristics of the display device are improved. A color which can beexhibited using the light-emitting elements of R, G, and B is limited toa color represented inside a triangle made by three points on thechromaticity diagram, each corresponding to the emission color of eachof the light-emitting elements. Therefore, another light-emittingelement of a color positioned outside the triangle on the chromaticitydiagram is added, whereby color reproduction characteristics of thedisplay device can be improved.

For example, a light-emitting element emitting the following color canbe used in addition to the light-emitting elements of R, G, and B in thebacklight 132: deep blue (DB) represented by a certain coordinate pointpositioned outside the triangle in a direction from the center of thechromaticity diagram toward a point on the chromaticity diagramcorresponding to the blue-light-emitting element B or deep red (DR)represented by a certain coordinate point positioned outside thetriangle in a direction from the center of the chromaticity diagramtoward a point on the chromaticity diagram corresponding to thered-light-emitting element R.

Next, a signal flow in the display device described in this embodimentwill be described.

An analog image signal 140 is input to the liquid crystal display device100 from an image signal supply source 101. The analog image signalincludes an image signal such as a signal corresponding to red (R), asignal corresponding to green (G), and a signal corresponding to blue(B).

The A/D converter circuit 102 converts the analog image signal into adigital image signal 141 (digital image signal Data) and outputs thesignal to the image processing circuit 110. When the image signal isconverted into a digital signal in advance, detection of a difference ofthe image signals that is to be performed later can be easily performed,which is preferable.

The image processing circuit 110 generates an LC image signal 142 and abacklight signal 143 from the digital image signal Data which is input.The LC image signal 142 is an image signal used for controlling thedisplay panel 120, and the backlight signal 143 is a signal used forcontrolling the backlight portion 130.

The memory circuit 111 provided in the image processing circuit 110includes a plurality of frame memories for storing image signals of aplurality of frames. The number of frame memories included in the memorycircuit 111 is not particularly limited as long as the image signals ofa plurality of frames can be stored. Note that the frame memory may beformed using a memory element such as dynamic random access memory(DRAM) or static random access memory (SRAM).

The number of frame memories is not particularly limited as long as theimage signal can be stored for each frame period. Further, the imagesignals stored in the frame memories are selectively read out by thecomparison circuit 112 and the display control circuit 113.

The comparison circuit 112 selectively reads out the image signals insuccessive frame periods stored in the memory circuit 111, compares theimage signals in the successive frame periods in each pixel, and detectsa difference thereof.

Depending on whether a difference is detected, operations in the displaycontrol circuit 113 and a selection circuit 115 are determined. When adifference is detected in any of the pixels by comparing the imagesignals in the comparison circuit 112, successive frame periods duringwhich the difference is detected are determined as a moving imageperiod. On the other hand, when a difference is not detected in all thepixels by comparing the image signals in the comparison circuit 112,successive frame periods during which no difference is detected aredetermined as a still image period. In other words, depending on whethera difference is detected by the comparison circuit 112, whether theimage signals in the successive frame periods are image signals fordisplaying a moving image or image signals for displaying a still imageis determined by the comparison circuit 112.

The difference obtained by the comparison may be set so as to bedetermined as a difference to be detected when it is over apredetermined level. The comparison circuit 112 may be set so as todetermine detection of a difference by the absolute value of thedifference regardless of the value of the difference.

Note that in this embodiment, a still image or a moving image isdetermined by detecting a difference between the image signals insuccessive frame periods with the comparison circuit 112; however, asignal used for switching between the still image and the moving imagemay be supplied externally, so that the still image or the moving imagemay be displayed in accordance with the switching signal.

Note that by switching of a plurality of images which is time-dividedinto a plurality of frames at high speed, the images are recognized as amotion image by human eyes. Specifically, by switching of images atleast 60 times (60 frames) per second, the images are recognized as amoving image with less flicker by human eyes. In contrast, unlike amoving image, a still image refers to image signals which do not changein successive frame periods, for example, in an n-th frame and an(n+1)th frame though a plurality of images which is time-divided into aplurality of frame periods is switched at high speed.

The selection circuit 115 includes a plurality of switches, for example,switches formed using transistors. The selection circuit 115 selects theimage signals from the frame memories in the memory circuit 111 wherethe image signals are stored, and the selection circuit 115 outputs theimage signals to the display control circuit 113 when a difference isdetected by calculation with the comparison circuit 112, that is, whenimages displayed in successive frame periods are a moving image.

Note that the selection circuit 115 does not output the image signals tothe display control circuit 113 when a difference between the imagesignals is not detected by calculation with the comparison circuit 112,that is, when images displayed in successive frame periods are a stillimage. When a still image is displayed, the selection circuit 115 doesnot output the image signals from the frame memories to the displaycontrol circuit 113, resulting in a reduction in power consumption.

In the display device of this embodiment, a mode performed in such a waythat the comparison circuit 112 determines the image signals as a stillimage is described as the still-image mode, and a mode performed in sucha way that the comparison circuit 112 determines the image signals as amoving image is described as the moving-image mode.

The image processing circuit described in this embodiment may have amode-switching function. The mode-switching function is a function ofswitching between the moving-image mode and the still-image mode in sucha manner that a user of the display device selects an operation mode ofthe display device by hand or using an external connection device.

Accordingly, the display device described in this embodiment may includea mode-switching circuit. The mode-switching circuit is connected to theselection circuit 115. The mode-switching circuit is an input means forswitching the operation mode of the display device by a user of thedisplay device by hand or using an external connection device.

The selection circuit 115 can output the image signals to the displaycontrol circuit 113 in accordance with signals input from themode-switching circuit.

For example, in the case where a user switches an operation mode and amode-switching signal is input to the selection circuit 115 from themode-switching circuit while an operation is performed in a still-imagedisplay mode, even when the comparison circuit 112 does not detect thedifference of image signals in successive frame periods, the user cancarry out a mode in which an image signal which is input is sequentiallyoutput to the display control circuit 113, that is, a moving-image mode.In the case where a user switches an operation mode and a mode-switchingsignal is input to the selection circuit 115 from the mode-switchingcircuit while an operation is performed in a moving-image mode, evenwhen the comparison circuit 112 detects the difference of the imagesignals in successive frame periods, the user can carry out a mode inwhich only an image signal of one selected frame is output, that is, astill-image mode.

Therefore, when the display device of this embodiment is operated in themoving-image mode, one frame among images which are time-divided into aplurality of frames is displayed as a still image.

The display control circuit 113 optimizes an image signal selected bythe selection circuit 115 in accordance with detection of the differencein the comparison circuit 112 for the display panel 120 and thebacklight portion 130.

For example, even when the digital image signal 141 is formed of signalsof R, G, and B, it is preferable that the image signal be optimized inaccordance with the light-emitting properties of the light-emittingelements of R, G, and B included in the backlight 132. In the case wherethe backlight 132 is provided with a light-emitting element other thanthe light-emitting elements of R, G, and B, the display control circuit113 generates a signal used for driving the light-emitting element fromthe original image signal, and color reproduction characteristics of thedisplay device is optimized.

For example, in the case where a digital image signal Data(1) formed ofR, G, and B is converted into a digital image signal Data(4) suitablefor the backlight 132 provided with light-emitting elements of fivecolors (R, G, B, DR, and DB), the display control circuit 113 generatesa digital image signal Data(2) expressed using the light-emittingelements of DR and DB from the original digital image signal Data(1). Atthe same time, the digital image signal Data(2) expressed using thelight-emitting elements of DR and DB is extracted from the originaldigital image signal Data(1), and a digital image signal Data(3) isgenerated. Next, the digital image signal Data(4), which is optimizedfor the backlight 132 provided with the light-emitting elements of fivecolors (R, G, B, DR, and DB) and which includes the digital image signalData(2) expressed using the light-emitting elements of DR and DB and thedigital image signal Data(3) expressed using the light-emitting elementsof R, G, and B, is generated.

The display device described in this embodiment includes the firstsub-pixel 123 a connected to the first signal line and the secondsub-pixel 123 b connected to the second signal line. The display controlcircuit 113 determines a signal line which outputs an image signal.

Specifically, when the comparison circuit 112 determines that an imagesignal is a moving image, the display control circuit 113 outputs theimage signal to the first sub-pixel 123 a. When the comparison circuit112 determines that an image signals is a still image, the displaycontrol circuit 113 outputs the image signal to the second sub-pixel 123b.

The field sequential signal generator circuit 114 controls the drivercircuit 121 of the display panel 120 and the backlight control circuit131 of the backlight portion 130 based on an image signal generated bythe display control circuit 113.

In addition, the field sequential signal generator circuit 114 controlsthe switching of supply and stop of control signals such as a startpulse SP and a clock signal CK which are used for synchronizing thedisplay panel 120 and the backlight portion 130.

Next, a method by which the field sequential signal generator circuit114 controls the driver circuit 121 of the display panel 120 and thebacklight control circuit 131 of the backlight portion 130 will bedescribed. The operation of the field sequential signal generatorcircuit 114 differs between the case where the comparison circuit 112determines that an image signal is a moving image and the case where thecomparison circuit 112 determines that an image signals is a stillimage. In this embodiment, the image signal is formed of R, G, and B,and the backlight 132 includes light-emitting elements (specifically,LEDs) of R, G, and B.

First, the operation of the field sequential signal generator circuit114 when the comparison circuit 112 determines that an image signal is amoving image will be described. In the field sequential signalgeneration circuit 114, an image signal including a moving image isprocessed in a moving-image mode. Specifically, the field sequentialsignal generation circuit 114 compresses each image signal optimized bythe display control circuit 113 by 1/(3n) with respect to the time axis.Note that n corresponds to the n used in the case where one frame isdivided into n sub-frames. Then, a field sequential color image signal(e.g., R1, G1, B1, R2, G2, and B2) is supplied to the driver circuit121, which correspond to R, G and B compressed by 1/(3n) with respect tothe time axis.

The field sequential signal generator circuit 114 supplies the backlightsignal 143 to the backlight 132. The backlight signal 143 makes thelight-emitting elements of R, G, and B provided in the backlight 132emit light, and the backlight signal 143 is paired with the fieldsequential color image signal which correspond to R, G, and B.

The display panel 120 and the backlight portion 130 operate insynchronization with a synchronization signal generated by the fieldsequential signal generator circuit 114, whereby a moving image isdisplayed.

On the other hand, when the comparison circuit 112 determines that animage signal is a still image, the field sequential signal generatorcircuit 114 does not generate the field sequential color image signalbut supplies still image data for one frame to the driver circuit 121 ofthe display panel 120.

Then, the field sequential signal generator circuit 114 stops the supplyof the image signal and the control signals to the driver circuit 121and the backlight control circuit 131.

The display device described in this embodiment may include aphotometric circuit. The display device provided with the photometriccircuit can detect the brightness of the environment where the displaydevice is put. As a result, the display control circuit 113 connected tothe photometric circuit can change a driving method of the display panel120 in accordance with a signal input from the photometric circuit.

For example, when the photometric circuit detects the display devicedescribed in this embodiment which is used in a dim environment, thedisplay control circuit 113 outputs an image signal to the firstsub-pixel 123 a and the backlight 132 is turned on even when thecomparison circuit 112 determines that an image signal is a still image.Since the first sub-pixel 123 a includes the light-transmitting pixelelectrode, a still image with high visibility can be provided using thebacklight.

For example, when the photometric circuit detects the display devicedescribed in this embodiment which is used under extremely brightexternal light (e.g., under direct sunlight outdoors), the displaycontrol circuit 113 outputs an image signal to the second sub-pixel 123b even when the comparison circuit 112 determines that an image signalsis a moving image. Since the second sub-pixel 123 b includes a pixelelectrode which reflects incident light through the liquid crystallayer, a still image with high visibility can be provided even underextremely bright external light.

In a period in which a still image is displayed using the structure ofthis embodiment, frequent writings of the image signal can be reduced.In addition, power consumption is extremely low because the still imagecan be displayed without use of the backlight.

The display device described in this embodiment can display not only astill image with reduced power consumption but also a full-color imageand a moving image without use of a color filter. Since the color filterdoes not absorb light of the backlight, light use efficiency is high,and power consumption is suppressed even when the full-color image andthe moving image are displayed.

When human eyes see an image formed by writing the image signal pluraltimes, the human eyes see images which are switched plural times, whichmight cause eye strain. As described in this embodiment, the number ofwritings of the image signal is reduced, whereby there is an effect ofreducing eye strain.

This embodiment can be combined with any of the other embodiments inthis specification, as appropriate.

Embodiment 2

In this embodiment, a driving method of a liquid crystal display devicewill be described using a pixel connection diagram, a timing chart, andthe like. First, FIG. 2 is a schematic view of a display panel of aliquid crystal display device. In FIG. 2, the display panel includes apixel portion 151, a first scan line 152 (also referred to as a gateline), a first signal line 153 (also referred to as a data line), asecond scan line 154, a second signal line 155, a pixel 156, a commonelectrode 169, a capacitor line 170, a first scan line driver circuit157, a first signal line driver circuit 158, a second scan line drivercircuit 159, and a second signal line driver circuit 160.

The pixel 156 is roughly divided into a light-transmitting electrodeportion 161 and a reflective electrode portion 162. Thelight-transmitting electrode portion 161 includes a pixel transistor163, a liquid crystal element 164, and a capacitor 165. A gate of thepixel transistor 163 is connected to the first scan line 152, a firstterminal serving as one of a source and a drain of the pixel transistor163 is connected to the first signal line 153, and a second terminalserving as the other of the source and the drain of the pixel transistor163 is connected to one electrode of the liquid crystal element 164 anda first electrode of the capacitor 165. The other electrode of theliquid crystal element 164 is connected to the common electrode 169. Asecond electrode of the capacitor 165 is connected to the capacitor line170.

The reflective electrode portion 162 includes a pixel transistor 166, aliquid crystal element 167, and a capacitor 168. A gate of the pixeltransistor 166 is connected to the second scan line 154, a firstterminal serving as one of a source and a drain of the pixel transistor166 is connected to the second signal line 155, a second terminalserving as the other of the source and the drain of the pixel transistor166 is connected to one electrode of the liquid crystal element 167 anda first electrode of the capacitor 168. The other electrode of theliquid crystal element 167 is connected to the common electrode 169. Asecond electrode of the capacitor 168 is connected to the capacitor line170.

In FIG. 2, the first scan line 152 and the second scan line 154 areseparately driven by the first scan line driver circuit 157 and thesecond scan line driver circuit 159, respectively. Respective imagesignals (hereinafter referred to as first data and second data) aresupplied to the first signal line 153 and the second signal line 155 bythe first signal line driver circuit 158 and the second signal linedriver circuit 160, respectively. Grayscales based on different imagesignals are controlled in the liquid crystal element 164 of thelight-transmitting electrode portion 161 and the liquid crystal element167 of the reflective electrode portion 162.

The pixel transistor 163 and the pixel transistor 166 are preferablyformed using thin film transistors (hereinafter also referred to asTFTs) having a thin oxide semiconductor layer.

Note that a thin film transistor is an element having at least threeterminals of gate, drain, and source. The thin film transistor includesa channel region between a drain region and a source region, and currentcan flow through the drain region, the channel region, and the sourceregion. Here, since the source and the drain may change depending on thestructure, the operating condition, and the like of the transistor, itis difficult to define which is a source or a drain. Therefore, in thisdocument (the specification, the claims, the drawings, and the like), aregion functioning as a source and a drain is not called the source orthe drain in some cases. In such a case, for example, one of the sourceand the drain may be referred to as a first terminal and the otherthereof may be referred to as a second terminal. Alternatively, one ofthe source and the drain may be referred to as a first electrode and theother thereof may be referred to as a second electrode. Furtheralternatively, one of the source and the drain may be referred to as asource region and the other thereof may be referred to as a drainregion.

The first scan line driver circuit 157, the first signal line drivercircuit 158, the second scan line driver circuit 159, and the secondsignal line driver circuit 160 are preferably provided over thesubstrate over which the pixel portion 151 is formed; however, these arenot necessarily formed over the substrate over which the pixel portion151 is formed. When the first scan line driver circuit 157, the firstsignal line driver circuit 158, the second scan line driver circuit 159,and the second signal line driver circuit 160 are provided over thesubstrate over which the pixel portion 151 is formed, the number of theconnection terminals for connection to the outside and the size of theliquid crystal display device can be reduced.

Note that the pixels 156 are provided (arranged) in matrix. Here,description that pixels are provided (arranged) in matrix includes thecase where the pixels are arranged in a straight line and the case wherethe pixels are arranged in a jagged line, in a longitudinal direction ora lateral direction.

Note that when it is explicitly described that “A and B are connected,”the case where A and B are electrically connected, the case where A andB are functionally connected, and the case where A and B are directlyconnected are included therein.

Next, the operation of the display panel together with the operation ofthe backlight will be described with reference to FIG. 3A. As describedin the above embodiment, the operation of the display panel is roughlydivided into a moving-image display period 301 and a still-image displayperiod 302.

The cycle of one frame period (or frame frequency) is preferably lessthan or equal to 1/60 sec (more than or equal to 60 Hz) in themoving-image display period 301. The frame frequency is increased, sothat flickering is not sensed by a viewer of an image. In thestill-image display period 302, the cycle of one frame period isextremely long, for example, longer than or equal to one minute (lessthan or equal to 0.017 Hz), so that eye strain can be reduced comparedto the case where the same image is switched plural times.

When an oxide semiconductor is used for a semiconductor layer of thepixel transistor 163 and the pixel transistor 166, the off-state currentcan be reduced. Accordingly, an electrical signal such as the imagesignal can be held for a longer period in the pixel, and a writinginterval can be set long. Therefore, the cycle of one frame period canbe increased, and the frequency of refresh operation in the still-imagedisplay period 302 can be reduced, whereby an effect of suppressingpower consumption can be further increased.

As described in the above embodiment, in the moving-image display period301 illustrated in FIG. 3A, driver circuit control signals fordisplaying a moving image by field sequential driving are supplied tothe first scan line driver circuit 157 and the first signal line drivercircuit 158 (hereinafter referred to as the first driver circuits), anddriver circuit control signals for displaying black in each pixel aresupplied to the second scan line driver circuit 159 and the secondsignal line driver circuit 160 (hereinafter referred to as the seconddriver circuits), whereby the first driver circuits and the seconddriver circuits operate. In the moving-image display period 301illustrated in FIG. 3A, the backlight signal 143 for performing colordisplay is supplied to the backlight by the field sequential driving, sothat the backlight operates. Then, a color moving image can be displayedon the display panel.

As described in the above embodiment, in the still-image display period302 illustrated in FIG. 3A, driver circuit control signals for writingthe image signals of a still image are supplied to the second drivercircuits because a monochrome grayscale (in the diagram, described asBK/W) is displayed due to transmission or non-transmission of reflectedlight, whereby the second driver circuits operate. When the drivercircuit control signals are not supplied to the second driver circuitsin the period other than the period of writing the image signals, powerconsumption can be reduced. In the still-image display period 302illustrated in FIG. 3A, display comes to be visible utilizing reflectedexternal light; therefore, the backlight is not operated by thebacklight control signals. Then, a still image with a monochromegrayscale can be displayed on the display panel.

Next, the moving-image display period 301 and the still-image displayperiod 302 of FIG. 3A will be described in details with reference totiming charts of FIG. 3B and FIG. 3C, respectively. The timing chartsillustrated in FIG. 3B and FIG. 3C are exaggerated for description, andsignals do not operate in synchronization, except for the case wherethere is specific description.

First, FIG. 3B will be described. FIG. 3B illustrates clock signals GCK(in the diagram, GCK1 and GCK2) which are supplied to the first scanline driver circuit 157 and the second scan line driver circuit 159,start pulses GSP (in the diagram, GSP1 and GSP2) which are supplied tothe first scan line driver circuit 157 and the second scan line drivercircuit 159, clock signals SCK (in the diagram, SCK1 and SCK2) which aresupplied to the first signal line driver circuit 158 and the secondsignal line driver circuit 160, start pulses SSP (in the diagram, SSP1and SSP2) which are supplied to the first signal line driver circuit 158and the second signal line driver circuit 160, the first data, thesecond data, and a lighting state of the backlight in the moving-imagedisplay period 301 as an example. As the backlight, a structure in whichthree colors of RGB are sequentially emitted as an example of aplurality of light-emitting elements will be described. Low powerconsumption and life extension can be attempted using an LED as thebacklight.

In the moving-image display period 301, each of the clock signals GCK1and GCK2 serves as a clock signal which is always supplied. Each of thestart pulses GSP1 and GSP2 serves as a pulse corresponding to verticalsynchronization frequency. Each of the clock signals SCK1 and SCK2serves as a clock signal which is always supplied. Each of the startpulses SSP1 and SSP2 serves as a pulse corresponding to one gateselection period. A moving image is displayed by field sequentialdriving in the moving-image display period 301. Therefore, image signalsare changed as follows: an image signal for displaying R (red) iswritten to each pixel, the backlight of R is turned on, an image signalfor displaying G (green) is written to each pixel, the backlight of G isturned on, an image signal for displaying B (blue) is written to eachpixel, and the backlight of B is turned on. The above operation isrepeated to change the image signals, whereby a viewer can see colordisplay of a moving image. In the moving-image display period 301, thesecond data is an image signal for displaying a grayscale of BK (black)and is written to the reflective electrode portion 162 of the pixel 156.When the second data is used as an image signal for displaying black,the reflective electrode portion 162 is irradiated with external light.Therefore, the visibility problem of a moving image of thelight-transmitting electrode portion 161 such that light leakage occursand visibility is reduced can be remedied.

Next, FIG. 3C will be described. In FIG. 3C, the still-image displayperiod 302 is divided into a still-image writing period 303 and astill-image holding period 304.

In the still-image writing period 303, the clock signal GCK2 supplied tothe second scan line driver circuit 159 serves as a clock signal forwriting for one screen. The start pulse GSP2 supplied to the second scanline driver circuit 159 serves as a pulse for writing for one screen.The clock signal SCK2 supplied to the second signal line driver circuit160 serves as a clock signal for writing for one screen. The start pulseSSP2 supplied to the second signal line driver circuit 160 serves as apulse for writing for one screen. In the still-image writing period 303,a still image is displayed using the image signal BK/W for displaying amonochrome grayscale utilizing reflected light; therefore, the backlightfor color display is not turned on.

In the still-image holding period 304, supply of the clock signals GCK1and GCK2, the start pulses GSP1 and GSP2, the clock signals SCK1 andSCK2, and the start pulses SSP1 and SSP2 which are used for driving thefirst driver circuits and the second driver circuits is stopped.Therefore, in the still-image holding period 304, power consumption canbe reduced. In the still-image holding period 304, the image signalswritten to the pixel in the still-image writing period 303 are held bythe pixel transistor with extremely low off-state current; therefore, acolor still image can be held for longer than or equal to one minute. Inthe still-image holding period 304, before the image signal held in thecapacitor is changed as a given period passes, another still-imagewriting period 303 is provided, and an image signal which is the same asthe image signal of the previous period is written (refresh operation),and the still-image holding period 304 may be provided again.

In the liquid crystal display device described in this embodiment, powerconsumption can be reduced when a still image is displayed.

This embodiment can be implemented in combination with the structuredescribed in Embodiment 1, as appropriate.

Embodiment 3

In this embodiment, a driving method which is different from the drivingmethod of the liquid crystal display device described in Embodiment 2will be described with reference to a timing chart and the like. First,a driving method of the backlight in the moving-image display period 301described in Embodiment 2 will be described using a timing chart in FIG.4A.

The timing chart in FIG. 4A is different from that of FIG. 3B in that anon-lighting period (BL of FIG. 4A) of the backlight is provided afterthe backlight is turned on, following the writing of the image signal.The non-lighting period of the backlight is provided before writing ofthe next image signal, so that a flicker of color or the like can bereduced and visibility can be improved.

FIG. 4B illustrates a structure different from that in FIG. 4A. Thetiming chart in FIG. 4B is different from that of FIG. 4A in that a B(blue) light-emitting period is provided instead of the non-lightingperiod BL of the backlight. In a manner similar to that of the casewhere the non-lighting period is provided, the blue light-emittingperiod is provided before writing of the next image signal, so that aflicker of color or the like can be reduced and visibility can beimproved.

In Embodiment 2, the example in which three colors of RGB are used isdescribed as an example of a plurality of light-emitting elements usedfor the backlight; however, another structure may be used. As anexample, the backlight may be controlled using a light-emitting element311 of five colors as illustrated in FIG. 5A.

FIG. 5A illustrates the light-emitting element 311 including a first redlight-emitting element R1, a second red light-emitting element R2, agreen light-emitting element G a first blue light-emitting element B1,and a second blue light-emitting element B2 as an example. Next in FIG.5B, control of the lighting of the backlight illustrated in FIG. 5A inthe moving-image display period 301 described in Embodiment 2 will bedescribed, similarly to the descriptions of FIGS. 4A and 4B.

In FIG. 5B, the first red light-emitting element R1 and the first bluelight-emitting element B1 emit light as the lighting of the backlightafter the writing of the R image signal. Next, the green light-emittingelement G and the second blue light-emitting element B2 emit light asthe lighting of the backlight after the writing of the G image signal.Then, the first blue light-emitting element B1 and the second bluelight-emitting element B2 emit light as the lighting of the backlightafter the writing of the B image signal. Next, the second redlight-emitting element R2 and the second blue light-emitting element B2emit light as the lighting of the backlight after the writing of the Rimage signal. Then, the green light-emitting element G and the firstblue light-emitting element B1 emit light as the lighting of thebacklight after the writing of the G image signal. Next, the second bluelight-emitting element B2 and the first blue light-emitting element B1emit light as the lighting of the backlight after the writing of the Bimage signal.

With the structure of FIG. 5B, the blue light-emitting period can beprovided in a period in which color elements of RGB are switched;therefore, an effect which is similar to that of FIG. 4B can beobtained. The first red light-emitting element R1 and the second redlight-emitting element R2 can be formed using materials of differentcolor coordinates and the first blue light-emitting element B1 and thesecond blue light-emitting element B2 can be formed using materials ofdifferent color coordinates; accordingly, color representation range canbe expanded in color display.

In the liquid crystal display device described in this embodiment, lowerpower consumption can be achieved when a still image is displayed.

This embodiment can be implemented in combination with the structuredescribed in Embodiment 1, as appropriate.

Embodiment 4

FIG. 6 illustrates a structure of a liquid crystal display module 190.The liquid crystal display module 190 includes the backlight portion130, the display panel 120 in which liquid crystal elements are arrangedin matrix, and a polarizing plate 125 a and a polarizing plate 125 bwhich are provided with the display panel 120 positioned therebetween.In the backlight portion 130, light-emitting elements, for example, LEDs(133R, 133G, and 133B) of three primary colors are arranged in matrix,and the backlight portion 130 may include a diffusion plate 134 providedbetween the display panel 120 and the light-emitting element. Inaddition, a flexible printed circuit (FPC) 126 serving as an externalinput terminal is electrically connected to a terminal portion providedin the display panel 120.

In FIG. 6, light 135 of three colors is schematically denoted by arrows(R, G, and B). Pulsed light of different colors sequentially emittedfrom the backlight portion 130 is modulated by the liquid crystalelement of the display panel 120 which is operated in synchronizationwith the backlight portion 130 to reach a viewer through the liquidcrystal display module 190. The viewer perceives light which issequentially emitted to be an image.

Further, FIG. 6 schematically illustrates a state in which externallight 139 is transmitted through the liquid crystal element over thedisplay panel 120 and reflected by an electrode below the liquid crystalelement. The intensity of the light transmitted through the liquidcrystal element is modulated by an image signal; therefore, a viewer canperceive an image also by the reflected light of the external light 139.

FIG. 7A is a plan view of a display region, and FIG. 7B illustrates anequivalent circuit. FIGS. 7A and 7B each illustrate one pixel. FIG. 8 isa cross-sectional view taken along lines V1-V2, W1-W2, and X1-X2 of FIG.7A.

In FIGS. 7A and 7B, a plurality of source wiring layers (including asource or drain electrode layer 555 b and a source or drain electrodelayer 565 b) is arranged in parallel (extends in the vertical directionin the drawing) to be spaced from each other. A plurality of gate wiringlayers (including a gate electrode layer 551) is provided to extend in adirection generally perpendicular to the source wiring layers (thehorizontal direction in the drawing) and to be spaced from each other.Capacitor wiring layers are arranged adjacent to the plurality of gatewiring layers and extend in a direction generally parallel to the gatewiring layers, that is, in a direction generally perpendicular to thesource wiring layers (in the horizontal direction in the drawing).

The liquid crystal display device in FIGS. 7A and 7B and FIG. 8 is asemi-transmissive liquid crystal display device in which a pixel regionincludes a reflective region 498 and a transmissive region 499. In thereflective region 498, a reflective electrode layer 577 is formed as apixel electrode layer, and in the transmissive region 499, a transparentelectrode layer 576 is formed as a pixel electrode layer. As illustratedin FIGS. 7A and 7B and FIG. 8, when the reflective electrode layer 577and the transparent electrode layer 576 are stacked in such a way thatan end portion of the reflective electrode layer 577 overlaps with anend portion of the transparent electrode layer 576 with an insulatingfilm 571 interposed therebetween, a display region can be efficientlyprovided in the pixel region. Note that an example in which thetransparent electrode layer 576, the insulating film 571, and thereflective electrode layer 577 are stacked in that order over aninterlayer film 413 is illustrated in FIG. 8; however, a structure inwhich the reflective electrode layer 577, the insulating film 571, andthe transparent electrode layer 576 are stacked in that order over theinterlayer film 413 may be employed.

The equivalent circuit in FIG. 7B includes, in one pixel, a transistor560 which is electrically connected to the reflective electrode layer577 and the source or drain electrode layer 565 b, and a transistor 550which is electrically connected to the transparent electrode layer 576and a source or drain electrode layer 555 b. The transistor 560 is atransistor for the reflective region, which controls on/off of thereflective region, and the transistor 550 is a transistor for thetransmissive region, which controls on/off of the transmissive region.

Insulating films 407 and 409 and the interlayer film 413 are providedover the transistors 550 and 560. The transistor 550 is electricallyconnected to the transparent electrode layer 576 and the transistor 560is electrically connected to the reflective electrode layer 577 in theirrespective openings (contact holes) formed in the insulating films 407and 409 and the interlayer film 413.

As illustrated in FIG. 8, a common electrode layer 448 (also referred toas a counter electrode layer) is formed on a second substrate 442 andfaces the transparent electrode layer 576 and the reflective electrodelayer 577 over a first substrate 441 with a liquid crystal layer 444provided therebetween. Note that in the liquid crystal display device inFIGS. 7A and 7B and FIG. 8, an alignment film 460 a is provided betweenthe transparent electrode layer 576 and the reflective electrode layer577, and the liquid crystal layer 444. An alignment film 460 b isprovided between the common electrode layer 448 and the liquid crystallayer 444. The alignment films 460 a and 460 b are insulating layershaving a function of controlling the alignment of liquid crystal andtherefore, are not necessarily provided depending on a material of theliquid crystal.

The transistors 550 and 560 are examples of bottom-gateinverted-staggered transistors. The transistor 550 includes the gateelectrode layer 551, a gate insulating layer 402, a semiconductor layer553, the source or drain electrode layer 555 a, and the source or drainelectrode layer 555 b. The transistor 560 includes the gate electrodelayer 551, the gate insulating layer 402, a semiconductor layer 563, asource or drain electrode layer 565 a, and the source or drain electrodelayer 565 b. The transistors 550 and 560 each have a capacitor. In thereflective region 498 as illustrated in FIG. 8, the capacitor wiringlayer 558 which is formed in the same step as the gate electrode layer551, the gate insulating layer 402, and a conductive layer 579 which isformed in the same step as the source or drain electrode layers 555 a,555 b, 565 a, and 565 b are stacked to form the capacitor. Note that itis preferable to form a wiring layer 580 formed in the same step as thereflective electrode layer 577 which is formed using a reflectiveconductive film of aluminum (Al), silver (Ag), or the like so as tocover the capacitor wiring layer 558.

The semi-transmissive liquid crystal display device in this embodimentdisplays a color moving image in the transmissive region 499 by controlof turning on and off the transistor 550 and a monochrome (black andwhite) still image in the reflective region 498 by control of turning onand off the transistor 560. The transistor 550 and the transistor 560are separately operated, so that display of the reflective region 498and display of the transmissive region 499 can be controlledindependently.

In the transmissive region 499, display is performed by incident lightfrom a backlight provided on the first substrate 441 side. Thelight-emitting diodes (LEDs) of RGB are used for the backlight, wherebycolor display can be performed. In this embodiment, a successiveadditive color mixing system (field sequential system) in which colordisplay is performed by time division using the light-emitting diodes(LEDs) is adopted.

On the other hand, in the reflective region 498, display is performed byreflecting external light incident from the second substrate 442 side bythe reflective electrode layer 577.

Examples in which the reflective electrode layer 577 is formed to haveunevenness in the liquid crystal display device are illustrated in FIG.9 and FIG. 10. FIG. 9 illustrates an example in which a surface of theinterlayer film 413 in the reflective region 498 is formed to have anuneven shape so that the reflective electrode layer 577 has an unevenshape. The uneven shape of the surface of the interlayer film 413 may beformed by performing selective etching. For example, the interlayer film413 having the uneven shape can be formed, for example, by performing aphotolithography step on a photosensitive organic resin. FIG. 10illustrates an example in which projected structures are provided overthe interlayer film 413 in the reflective region 498 so that thereflective electrode layer 577 has an uneven shape. Note that in FIG.10, the projected structures are formed by stacking an insulating layer480 and an insulating layer 482. For example, an inorganic insulatinglayer of silicon oxide, silicon nitride, or the like can be used as theinsulating layer 480, and an organic resin such as a polyimide resin oran acrylic resin can be used as the insulating layer 482. First, asilicon oxide film is formed over the interlayer film 413 by asputtering method, and a polyimide resin film is formed over the siliconoxide film by a coating method. The polyimide resin film is etched withthe use of the silicon oxide film as an etching stopper. The siliconoxide film is etched with the use of the etched polyimide resin layer asa mask, so that the projected structures including a stack of theinsulating layer 480 and the insulating layer 482 can be formed asillustrated in FIG. 10.

When the reflective electrode layer 577 has an uneven surface asillustrated in FIG. 9 and FIG. 10, incident light from the outside isirregularly reflected, so that more favorable display can be performed.Accordingly, the visibility of display is improved.

This embodiment can be freely combined with Embodiments 1 to 3.

Embodiment 5

In this embodiment, an example of a transistor which can be applied to aliquid crystal display device disclosed in this specification will bedescribed. There is no particular limitation on a structure of atransistor which can be applied to a liquid crystal display devicedisclosed in this specification. For example, a top-gate structure or abottom-gate structure such as a staggered type and a planar type can beused. The transistor may have a single-gate structure in which onechannel formation region is formed, a double-gate structure in which twochannel formation regions are formed, or a triple-gate structure inwhich three channel formation regions are formed. Alternatively, thetransistor may have a dual-gate structure including two gate electrodelayers positioned above and below a channel region with a gateinsulating layer provided therebetween. Note that examples of across-sectional structure of a transistor illustrated FIGS. 11A to 11Dare described below. Transistors illustrated in FIGS. 11A to 11D aretransistors including an oxide semiconductor as a semiconductor. Anadvantage of using an oxide semiconductor is that high mobility and lowoff-state current can be obtained in a relatively easy andlow-temperature process: however, it is needless to say that anothersemiconductor may be used.

A transistor 410 illustrated in FIG. 11A is one of bottom-gate thin filmtransistors, and is also referred to as an inverted-staggered thin filmtransistor.

The transistor 410 includes, over a substrate 400 having an insulatingsurface, a gate electrode layer 401, the gate insulating layer 402, anoxide semiconductor layer 403, a source electrode layer 405 a, and adrain electrode layer 405 b. In addition, the insulating film 407 whichcovers the transistor 410 and is stacked over the oxide semiconductorlayer 403 is provided. The insulating film 409 is provided over theinsulating film 407.

A transistor 420 illustrated in FIG. 11B is one of bottom-gate thin filmtransistors referred to as a channel-protective (channel-stop) thin filmtransistor and is also referred to as an inverted-staggered thin filmtransistor.

The transistor 420 includes, over the substrate 400 having an insulatingsurface, the gate electrode layer 401, the gate insulating layer 402,the oxide semiconductor layer 403, an insulating layer 427 functioningas a channel protective layer which covers a channel formation region ofthe oxide semiconductor layer 403, the source electrode layer 405 a, andthe drain electrode layer 405 b. The insulating film 409 is formed so asto cover the transistor 420.

A transistor 430 illustrated in FIG. 11C is a bottom-gate thin filmtransistor, and includes, over the substrate 400 having an insulatingsurface, the gate electrode layer 401, the gate insulating layer 402,the source electrode layer 405 a, the drain electrode layer 405 b, andthe oxide semiconductor layer 403. The insulating film 407 which coversthe transistor 430 and is in contact with the oxide semiconductor layer403 is provided. The insulating film 409 is provided over the insulatingfilm 407.

In the transistor 430, the gate insulating layer 402 is provided on andin contact with the substrate 400 and the gate electrode layer 401, andthe source electrode layer 405 a and the drain electrode layer 405 b areprovided on and in contact with the gate insulating layer 402. Further,the oxide semiconductor layer 403 is provided over the gate insulatinglayer 402, the source electrode layer 405 a, and the drain electrodelayer 405 b.

A thin film transistor 440 illustrated in FIG. 11D is one of top-gatethin film transistors. The transistor 440 includes, over the substrate400 having an insulating surface, an insulating layer 437, the oxidesemiconductor layer 403, the source electrode layer 405 a, the drainelectrode layer 405 b, the gate insulating layer 402, and the gateelectrode layer 401. A wiring layer 436 a and a wiring layer 436 b areprovided to be in contact with and electrically connected to the sourceelectrode layer 405 a and the drain electrode layer 405 b, respectively.

In this embodiment, as described above, the oxide semiconductor layer403 is used as a semiconductor layer. As an oxide semiconductor used forthe oxide semiconductor layer 403, an In—Sn—Ga—Zn—O-based oxidesemiconductor which is an oxide of four metal elements; anIn—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxidesemiconductor, an In—Al—Zn—O-based oxide semiconductor, aSn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxidesemiconductor, or a Sn-AI—Zn—O-based oxide semiconductor which areoxides of three metal elements; an In—Zn—O-based oxide semiconductor, aSn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor,a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxidesemiconductor, or an In—Mg—O-based oxide semiconductor which are oxidesof two metal elements; an In—O-based oxide semiconductor, a Sn—O-basedoxide semiconductor, or a Zn—O-based oxide semiconductor can be used.Further, SiO₂ may be contained in the above oxide semiconductor. Here,for example, an In—Ga—Zn—O-based oxide semiconductor is an oxideincluding at least In, Ga, and Zn, and there is no particular limitationon the composition ratio thereof. Further, the In—Ga—Zn—O-based oxidesemiconductor may contain an element other than In, Ga, and Zn.

For the oxide semiconductor layer 403, a thin film, represented by thechemical formula, InMO₃(ZnO)_(m) (m>0) can be used. Here, M representsone or more metal elements selected from Ga, Al, Mn, and Co. Forexample, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.

In the transistors 410, 420, 430, and 440 each including the oxidesemiconductor layer 403, a current value in an off state (off-statecurrent value) can be reduced. Therefore, an electrical signal of imagedata and the like can be held for a longer period, so that a writinginterval can be set long. Accordingly, frequency of refresh operationcan be reduced, which leads to an effect of suppressing powerconsumption.

Further, in the transistors 410, 420, 430, and 440 each including theoxide semiconductor layer 403, relatively high field-effect mobility canbe obtained, whereby high-speed operation is possible. Therefore, byusing any of the transistors in a pixel portion of a liquid crystaldisplay device, color separation can be suppressed and a high-qualityimage can be provided. Since the transistors can be separately formedover one substrate in a circuit portion and a pixel portion, the numberof components can be reduced in the liquid crystal display device.

Although there is no particular limitation on a substrate used for thesubstrate 400 having an insulating surface, a glass substrate of bariumborosilicate glass, aluminoborosilicate glass, or the like is used.

In the bottom-gate transistors 410, 420, and 430, an insulating filmserving as a base film may be provided between the substrate and thegate electrode layer. The base film has a function of preventingdiffusion of an impurity element from the substrate, and can be formedto have a single-layer or stacked-layer structure using one or morefilms selected from a silicon nitride film, a silicon oxide film, asilicon nitride oxide film, and a silicon oxynitride film.

The gate electrode layer 401 can be formed to have a single-layer orstacked-layer structure using a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, orscandium, or an alloy material which contains any of these materials asits main component.

The gate insulating layer 402 can be formed to have a single-layer orstacked-layer structure using any of a silicon oxide layer, a siliconnitride layer, a silicon oxynitride layer, a silicon nitride oxidelayer, an aluminum oxide layer, an aluminum nitride layer, an aluminumoxynitride layer, an aluminum nitride oxide layer, and a hafnium oxidelayer by a plasma CVD method, a sputtering method, or the like. Forexample, by a plasma CVD method, a silicon nitride layer (SiN_(y) (y>0))with a thickness of greater than or equal to 50 nm and less than orequal to 200 nm is formed as a first gate insulating layer, and asilicon oxide layer (SiO_(x) (x>0)) with a thickness of greater than orequal to 5 nm and less than or equal to 300 nm is formed as a secondgate insulating layer over the first gate insulating layer, so that agate insulating layer with a total thickness of 200 nm is formed.

A conductive film used for the source electrode layer 405 a and thedrain electrode layer 405 b can be formed using an element selected fromAl, Cr, Cu, Ta, Ti, Mo, and W, an alloy film containing any of theseelements, an alloy film containing a combination of any of theseelements, or the like. Alternatively, a structure may be employed inwhich a high-melting-point metal layer of Ti, Mo, W, or the like isstacked over and/or below a metal layer of Al, Cu, or the like. Inaddition, heat resistance can be improved by using an Al material towhich an element (Si, Nd, Sc, or the like) which prevents generation ofa hillock or a whisker in an Al film is added.

A material similar to that of the source electrode layer 405 a and thedrain electrode layer 405 b can be used for a conductive film such asthe wiring layer 436 a and the wiring layer 436 b which are connected tothe source electrode layer 405 a and the drain electrode layer 405 b,respectively.

Alternatively, the conductive film to be the source electrode layer 405a and drain electrode layer 405 b (including a wiring layer formed inthe same layer as the source and drain electrode layers) may be formedusing a conductive metal oxide. As the conductive metal oxide, indiumoxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), indium oxide-tinoxide alloy (In₂O₃—SnO₂, which is abbreviated to ITO), indium oxide-zincoxide alloy (In₂O₃—ZnO), or any of these metal oxide materials in whichsilicon oxide is contained can be used.

As the insulating film 407, and the insulating layers 427 and 437,typically, an inorganic insulating film such as a silicon oxide film, asilicon oxynitride film, an aluminum oxide film, or an aluminumoxynitride film can be used.

As the insulating film 409, an inorganic insulating film such as asilicon nitride film, an aluminum nitride film, a silicon nitride oxidefilm, or an aluminum nitride oxide film can be used.

In addition, a planarization insulating film may be formed over theinsulating film 409 in order to reduce surface unevenness due to thetransistor. As the planarization insulating film, an organic materialsuch as polyimide, acrylic, or benzocyclobutene can be used. Other thansuch organic materials, it is also possible to use a low-dielectricconstant material (a low-k material) or the like. Note that theplanarization insulating film may be formed by stacking a plurality ofinsulating films formed using these materials.

Thus, in this embodiment, a high-performance liquid crystal displaydevice can be provided by using a transistor including an oxidesemiconductor layer.

Embodiment 6

In this embodiment, an example of a transistor including an oxidesemiconductor layer and an example of a manufacturing method thereofwill be described in detail with reference to FIGS. 12A to 12E. The sameportions as those in the above embodiments and portions having functionssimilar to those of the portions in the above embodiments and stepssimilar to those in the above embodiments may be handled as in the aboveembodiments, and repeated description is omitted. In addition, detaileddescription of the same portions is not repeated.

FIGS. 12A to 12E illustrate an example of a cross-sectional structure ofa transistor. A transistor 510 illustrated in FIGS. 12A to 12E is abottom-gate inverted-staggered thin film transistor which is similar tothe transistor 410 illustrated in FIG. 11A.

An oxide semiconductor used for a semiconductor layer in this embodimentis an i-type (intrinsic) oxide semiconductor or a substantially i-type(intrinsic) oxide semiconductor. The i-type (intrinsic) oxidesemiconductor or substantially i-type (intrinsic) oxide semiconductor isobtained in such a manner that hydrogen, which is an n-type impurity, isremoved from an oxide semiconductor, and the oxide semiconductor ishighly purified so as to contain as few impurities that are not maincomponents of the oxide semiconductor as possible. In other words, ahighly-purified i-type (intrinsic) semiconductor or a semiconductorclose thereto is obtained not by adding impurities but by removingimpurities such as hydrogen or water as much as possible.

Accordingly, the oxide semiconductor layer included in the transistor510 is an oxide semiconductor layer which is highly purified and made tobe electrically i-type (intrinsic).

In addition, a highly-purified oxide semiconductor includes extremelyfew carriers (close to zero), and the carrier concentration thereof isless than 1×10¹⁴/cm³, preferably less than 1×10¹²/cm³, more preferablyless than 1×10¹¹/cm³.

Since the oxide semiconductor includes extremely few carriers, off-statecurrent of the transistor 510 can be reduced. The smaller the amount ofoff-state current is, the better.

Specifically, in the thin film transistor including the oxidesemiconductor layer, off-state current density per micrometer in achannel width at room temperature can be less than or equal to 10 aA/μm(1×10⁻¹⁷ A/μm), further less than or equal to 1 aA/μm (1×10⁻¹⁸ A/μm), orstill further less than or equal to 10 zA/μm (1×10⁻²⁰ A/μm).

When a transistor whose current value in an off state (anoff-state-current value) is extremely small is used as a transistor inthe pixel portion of Embodiment 1, the number of writings of refreshoperation in a still image region can be reduced.

In addition, in the transistor 510 including the oxide semiconductorlayer, the temperature dependence of on-state current is hardlyobserved, and off-state current remains extremely small.

Steps of manufacturing the transistor 510 over a substrate 505 aredescribed below with reference to FIGS. 12A to 12E.

First, a conductive film is formed over the substrate 505 having aninsulating surface; then, a gate electrode layer 511 is formed through afirst photolithography step. Note that a resist mask may be formed by aninkjet method. Formation of the resist mask by an inkjet method needs nophotomask; thus, manufacturing cost can be reduced.

As the substrate 505 having an insulating surface, a substrate similarto the substrate 400 described in Embodiment 5 can be used. In thisembodiment, a glass substrate is used as the substrate 505.

An insulating film serving as a base film may be provided between thesubstrate 505 and the gate electrode layer 511. The base film has afunction of preventing diffusion of an impurity element from thesubstrate 505, and can be formed with a single-layer or stacked-layerstructure using one or more of a silicon nitride film, a silicon oxidefilm, a silicon nitride oxide film, and a silicon oxynitride film.

In addition, the gate electrode layer 511 can be formed to have asingle-layer or stacked-layer structure using a metal material such asmolybdenum, titanium, tantalum, tungsten, aluminum, copper, neodymium,or scandium, or an alloy material which contains any of these materialsas its main component.

Next, a gate insulating layer 507 is formed over the gate electrodelayer 511. The gate insulating layer 507 can be formed to have asingle-layer or stacked-layer structure using a silicon oxide layer, asilicon nitride layer, a silicon oxynitride layer, a silicon nitrideoxide layer, an aluminum oxide layer, an aluminum nitride layer, analuminum oxynitride layer, an aluminum nitride oxide layer, or a hafniumoxide layer, by a plasma CVD method, a sputtering method, or the like.

As the oxide semiconductor in this embodiment, an oxide semiconductorwhich is made to be an i-type or substantially i-type by removingimpurities is used. Such a highly-purified oxide semiconductor isextremely sensitive to an interface level or interface charge;therefore, an interface between the oxide semiconductor layer and thegate insulating layer is important. For that reason, the gate insulatinglayer that is to be in contact with a highly-purified oxidesemiconductor needs to have high quality.

For example, a high-density plasma CVD method using microwaves (e.g., afrequency of 2.45 GHz) is preferably adopted because an insulating layercan be dense and can have high withstand voltage and high quality. Whena highly-purified oxide semiconductor and a high-quality gate insulatinglayer are in close contact with each other, the interface level can bereduced and interface characteristics can be favorable.

It is needless to say that another deposition method such as asputtering method or a plasma CVD method can be employed as long as ahigh-quality insulating layer can be formed as a gate insulating layer.Moreover, it is possible to use as the gate insulating layer aninsulating layer whose quality and characteristics of an interface withan oxide semiconductor are improved with heat treatment performed afterthe formation of the insulating layer. In any case, an insulating layerthat can reduce interface state density with an oxide semiconductor toform a favorable interface, as well as having favorable film quality asthe gate insulating layer, is formed.

Further, in order that hydrogen, a hydroxyl group, and moisture might becontained in the gate insulating layer 507 and an oxide semiconductorfilm 530 as little as possible, it is preferable that the substrate 505over which the gate electrode layer 511 is formed or the substrate 505over which layers up to and including the gate insulating layer 507 areformed be preheated in a preheating chamber of a sputtering apparatus aspretreatment for deposition of the oxide semiconductor film 530 so thatimpurities such as hydrogen and moisture adsorbed to the substrate 505are eliminated and exhaustion is performed. As an exhaustion unitprovided in the preheating chamber, a cryopump is preferable. Note thatthis preheating treatment can be omitted. This preheating step may besimilarly performed on the substrate 505 over which layers up to andincluding a source electrode layer 515 a and a drain electrode layer 515b are formed before formation of an insulating layer 516.

Next, the oxide semiconductor film 530 having a thickness of greaterthan or equal to 2 nm and less than or equal to 200 nm, preferablygreater than or equal to 5 nm and less than or equal to 30 nm is formedover the gate insulating layer 507 (see FIG. 12A).

Note that before the oxide semiconductor film 530 is formed by asputtering method, powder substances (also referred to as particles ordust) attached on a surface of the gate insulating layer 507 arepreferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering refers to amethod in which, without application of voltage to a target side, an RFpower source is used for application of voltage to a substrate side inan argon atmosphere and plasma is generated in the vicinity of thesubstrate to modify a surface. Note that instead of an argon atmosphere,a nitrogen atmosphere, a helium atmosphere, an oxygen atmosphere, or thelike may be used.

As an oxide semiconductor used for the oxide semiconductor film 530, anoxide semiconductor described in Embodiment 5, such as an oxide of fourmetal elements, an oxide of three metal elements, an oxide of two metalelements, an In—O-based oxide semiconductor, a Sn—O-based oxidesemiconductor, or a Zn—O-based oxide semiconductor can be used. Further,SiO₂ may be contained in the above oxide semiconductor. In thisembodiment, the oxide semiconductor film 530 is deposited by asputtering method with the use of an In—Ga—Zn—O-based oxidesemiconductor target. A cross-sectional view of this stage isillustrated in FIG. 12A. Alternatively, the oxide semiconductor film 530can be formed by a sputtering method in a rare gas (typically, argon)atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gasand oxygen.

As a target for manufacturing the oxide semiconductor film 530 by asputtering method, for example, a target having a compositional ratio ofIn₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio] can be used. Alternatively, a targethaving a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio] orIn₂O₃:Ga₂O₃:ZnO=1:1:4 [molar ratio] may be used. The filling rate of theoxide semiconductor target for film formation is higher than or equal to90% and lower than or equal to 100%, preferably, higher than or equal to95% and lower than or equal to 99.9%. With the use of the oxidesemiconductor target for film formation with high filling rate, thedeposited oxide semiconductor film has high density.

It is preferable that a high-purity gas in which an impurity such ashydrogen, water, a hydroxyl group, or hydride is removed be used as thesputtering gas for the deposition of the oxide semiconductor film 530.

The substrate is placed in a deposition chamber under reduced pressure,and the substrate temperature is set to a temperature higher than orequal to 100° C. and lower than or equal to 600° C., preferably higherthan or equal to 200° C. and lower than or equal to 400° C. Depositionis performed while the substrate is heated, whereby the concentration ofan impurity contained in the oxide semiconductor layer formed can bereduced. In addition, damage by sputtering can be reduced. Then,residual moisture in the deposition chamber is removed, a sputtering gasfrom which hydrogen and moisture are removed is introduced, and theabove-described target is used, so that the oxide semiconductor film 530is formed over the substrate 505. In order to remove the residualmoisture in the deposition chamber, an entrapment vacuum pump, forexample, a cryopump, an ion pump, or a titanium sublimation pump ispreferably used. The evacuation unit may be a turbo pump provided with acold trap. In the deposition chamber which is evacuated with thecryopump, for example, a hydrogen atom, a compound containing a hydrogenatom, such as water (H₂O), (more preferably, also a compound containinga carbon atom), and the like are removed, whereby the concentration ofan impurity in the oxide semiconductor film formed in the depositionchamber can be reduced.

As one example of the deposition condition, the distance between thesubstrate and the target is 100 mm, the pressure is 0.6 Pa, thedirect-current (DC) power source is 0.5 kW, and the atmosphere is anoxygen atmosphere (the proportion of the oxygen flow rate is 100%). Notethat a pulse direct current power source is preferable because powdersubstances (also referred to as particles or dust) generated indeposition can be reduced and the film thickness can be uniform.

Next, the oxide semiconductor film 530 is processed into anisland-shaped oxide semiconductor layer through a secondphotolithography step. A resist mask for forming the island-shaped oxidesemiconductor layer may be formed by an inkjet method. Formation of theresist mask by an inkjet method needs no photomask; thus, manufacturingcost can be reduced.

In the case where a contact hole is formed in the gate insulating layer507, a step of forming the contact hole can be performed at the sametime as processing of the oxide semiconductor film 530.

For the etching of the oxide semiconductor film 530, either one or bothof wet etching and dry etching may be employed. As an etchant used forwet etching of the oxide semiconductor film 530, for example, a mixedsolution of phosphoric acid, acetic acid, and nitric acid, or the likecan be used. In addition, ITO007N (produced by Kanto Chemical Co., Inc.)may also be used.

Next, first heat treatment is performed on the oxide semiconductorlayer. The oxide semiconductor layer can be dehydrated or dehydrogenatedby this first heat treatment. The temperature of the first heattreatment is higher than or equal to 400° C. and lower than or equal to750° C., or higher than or equal to 400° C. and lower than the strainpoint of the substrate. Here, the substrate is put in an electricfurnace which is a kind of heat treatment apparatus and heat treatmentis performed on the oxide semiconductor layer at 450° C. for one hour ina nitrogen atmosphere, and then, water or hydrogen is prevented fromentering the oxide semiconductor layer without exposure to the air;thus, an oxide semiconductor layer 531 is obtained (see FIG. 12B).

Note that a heat treatment apparatus is not limited to an electricalfurnace, and may include a device for heating an object to be processedby heat conduction or heat radiation from a heating element such as aresistance heating element. For example, a rapid thermal anneal (RTA)apparatus such as a gas rapid thermal anneal (GRTA) apparatus or a lamprapid thermal anneal (LRTA) apparatus can be used. An LRTA apparatus isan apparatus for heating an object to be processed by radiation of light(an electromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As the hightemperature gas, an inert gas which does not react with an object to betreated by heat treatment, such as nitrogen or a rare gas like argon, isused.

For example, as the first heat treatment, GRTA in which the substrate ismoved into an inert gas heated to a high temperature as high as 650° C.to 700° C., inclusive, heated for several minutes, and moved out of theinert gas heated to the high temperature may be performed.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in the atmosphere of nitrogen ora rare gas such as helium, neon, or argon. It is preferable that thepurity of nitrogen or the rare gas such as helium, neon, or argon whichis introduced into a heat treatment apparatus be set to be 6N (99.9999%)or more, preferably 7N (99.99999%) or more (that is, the impurityconcentration is 1 ppm or lower, preferably 0.1 ppm or lower).

Further, after the oxide semiconductor layer is heated in the first heattreatment, a high-purity oxygen gas, a high-purity N₂O gas, or anultra-dry air (the dew point is lower than or equal to −40° C.,preferably lower than or equal to −60° C.) may be introduced into thesame furnace. It is preferable that water, hydrogen, and the like be notcontained in an oxygen gas or an N₂O gas. The purity of the oxygen gasor the N₂O gas which is introduced into the heat treatment apparatus ispreferably 6N or more, more preferably 7N or more (that is, theconcentration of impurities in the oxygen gas or the N₂O gas ispreferably 1 ppm or lower, more preferably 0.1 ppm or lower). By theaction of the oxygen gas or the N₂O gas, oxygen which has been reducedat the same time as the step for removing impurities by dehydration ordehydrogenation is supplied, so that the oxide semiconductor layer canbe a highly-purified and electrically i-type (intrinsic) oxidesemiconductor.

In addition, the first heat treatment of the oxide semiconductor layercan also be performed on the oxide semiconductor film 530 which has notyet been processed into the island-shaped oxide semiconductor layer. Inthat case, the substrate is taken out from the heat apparatus after thefirst heat treatment; then a photolithography step is performed.

Note that the first heat treatment may be performed at any of thefollowing timings in addition to the above timing as long as afterdeposition of the oxide semiconductor layer: after a source electrodelayer and a drain electrode layer are formed over the oxidesemiconductor layer and after an insulating layer is formed over thesource electrode layer and the drain electrode layer.

Further, the step of forming the contact hole in the gate insulatinglayer 507 may be performed either before or after the first heattreatment is performed on the semiconductor film 530.

In addition, as the oxide semiconductor layer, an oxide semiconductorlayer having a crystal region with a large thickness (a single crystalregion), that is, a crystal region which is c-axis-alignedperpendicularly to a surface of the film may be formed by performingdeposition twice and heat treatment twice, even when any of an oxide, anitride, a metal, or the like is used for a material of a basecomponent. For example, a first oxide semiconductor film with athickness greater than or equal to 3 nm and less than or equal to 15 nmis deposited, and first heat treatment is performed in a nitrogen, anoxygen, a rare gas, or a dry air atmosphere at a temperature higher thanor equal to 450° C. and lower than or equal to 850° C. or preferablyhigher than or equal to 550° C. and lower than or equal to 750° C., sothat a first oxide semiconductor film having a crystal region (includinga plate-like crystal) in a region including a surface is formed. Then, asecond oxide semiconductor film which has a larger thickness than thefirst oxide semiconductor film is formed, and second heat treatment isperformed at a temperature higher than or equal to 450° C. and lowerthan or equal to 850° C. or preferably higher than or equal to 600° C.and lower than or equal to 700° C., so that crystal growth proceedsupward with the use of the first oxide semiconductor film as a seed ofthe crystal growth and the whole second oxide semiconductor film iscrystallized. In such a manner, the oxide semiconductor layer having acrystal region having a large thickness may be formed.

Next, a conductive film serving as the source and drain electrode layers(including a wiring formed in the same layer as the source and drainelectrode layers) is formed over the gate insulating layer 507 and theoxide semiconductor layer 531. As the conductive film serving as thesource and drain electrode layers, the material used for the sourceelectrode layer 405 a and the drain electrode layer 405 b which isdescribed in Embodiment 5 can be used.

A resist mask is formed over the conductive film through a thirdphotolithography step, and the source electrode layer 515 a and thedrain electrode layer 515 b are formed by selective etching; then, theresist mask is removed (see FIG. 12C).

Light exposure at the time of the formation of the resist mask in thethird photolithography step may be performed using ultraviolet light,KrF laser light, or ArF laser light. A channel length L of a transistorthat is completed later is determined by a distance between bottom endportions of the source electrode layer and the drain electrode layer,which are adjacent to each other over the oxide semiconductor layer 531.In the case where light exposure is performed for a channel length L ofless than 25 nm, the light exposure at the time of the formation of theresist mask in the third photolithography step may be performed usingextreme ultraviolet having an extremely short wavelength of severalnanometers to several tens of nanometers, inclusive. Light exposure withextreme ultraviolet leads to a high resolution and a large depth offocus. Thus, the channel length L of the transistor that is completedlater can be greater than or equal to 10 nm and less than or equal to1000 nm and the operation speed of a circuit can be increased andfurthermore the value of off-state current is extremely small, so thatlow power consumption can be achieved.

In order to reduce the number of photomasks used in a photolithographystep and reduce the number of photolithography steps, an etching stepmay be performed with the use of a multi-tone mask which is alight-exposure mask through which light is transmitted to have aplurality of intensities. A resist mask formed with the use of amulti-tone mask has a plurality of thicknesses and further can bechanged in shape by etching; therefore, the resist mask can be used in aplurality of etching steps for processing into different patterns.Therefore, a resist mask corresponding to at least two kinds or more ofdifferent patterns can be formed by one multi-tone mask. Thus, thenumber of light-exposure masks can be reduced and the number ofcorresponding photolithography steps can be also reduced, wherebysimplification of a process can be realized.

Note that it is preferable that etching conditions be optimized so asnot to etch and divide the oxide semiconductor layer 531 when theconductive film is etched. However, it is difficult to obtain etchingconditions in which only the conductive film is etched and the oxidesemiconductor layer 531 is not etched at all. In some cases, only partof the oxide semiconductor layer 531 is etched to be an oxidesemiconductor layer having a groove portion (a recessed portion) whenthe conductive film is etched.

In this embodiment, since the Ti film is used as the conductive film andthe In—Ga—Zn—O-based oxide semiconductor is used as the oxidesemiconductor layer 531, an ammonium hydroxide/hydrogen peroxide mixture(a mixed solution of ammonia water, water, and hydrogen peroxidesolution) is used as an etchant for the conductive film.

Next, by plasma treatment using a gas such as N₂O, N₂, or Ar, water orthe like adsorbed to a surface of an exposed portion of the oxidesemiconductor layer may be removed. In the case where the plasmatreatment is performed, the insulating layer 516 is formed withoutexposure to the air as a protective insulating film in contact with partof the oxide semiconductor layer.

The insulating layer 516 can be formed to a thickness of at least 1 nmby a method by which an impurity such as water or hydrogen does notenter the insulating layer 516, such as a sputtering method asappropriate. When hydrogen is contained in the insulating layer 516,entry of the hydrogen to the oxide semiconductor layer, or extraction ofoxygen in the oxide semiconductor layer by hydrogen may occur, therebycausing the backchannel of the oxide semiconductor layer to have lowerresistance (to be n-type), so that a parasitic channel might be formed.Therefore, it is important that a deposition method in which hydrogen isnot used is employed in order to form the insulating layer 516containing as little hydrogen as possible.

In this embodiment, a silicon oxide film is formed to a thickness of 200nm as the insulating layer 516 with a sputtering method. The substratetemperature in deposition may be higher than or equal to roomtemperature and lower than or equal to 300° C. and in this embodiment,is 100° C. The silicon oxide film can be deposited by a sputteringmethod in a rare gas (typically, argon) atmosphere, an oxygenatmosphere, or a mixed atmosphere containing a rare gas and oxygen. As atarget, a silicon oxide target or a silicon target may be used. Forexample, the silicon oxide film can be formed using a silicon target bya sputtering method in an atmosphere containing oxygen. As theinsulating layer 516 which is formed in contact with the oxidesemiconductor layer, an inorganic insulating film which does not includeimpurities such as moisture, a hydrogen ion, and OH⁻ and prevents entryof these from the outside is used. Typically, a silicon oxide film, asilicon oxynitride film, an aluminum oxide film, an aluminum oxynitridefilm, or the like can be used.

In order to remove residual moisture in the deposition chamber of theinsulating layer 516 as in the case of the deposition of the oxidesemiconductor film 530, an entrapment vacuum pump (such as a cryopump)is preferably used. When the insulating layer 516 is deposited in thedeposition chamber evacuated using a cryopump, the impurityconcentration in the insulating layer 516 can be reduced. In addition,as an exhaustion unit for removing the residual moisture in thedeposition chamber of the insulating layer 516, a turbo pump providedwith a cold trap may be used.

It is preferable that a high-purity gas in which an impurity such ashydrogen, water, a hydroxyl group, or hydride is removed be used as thesputtering gas for the deposition of the insulating layer 516.

Next, second heat treatment is performed in an inert gas atmosphere oroxygen gas atmosphere (preferably at a temperature higher than or equalto 200° C. and lower than or equal to 400° C., for example, higher thanor equal to 250° C. and lower than or equal to 350° C.). For example,the second heat treatment is performed in a nitrogen atmosphere at 250°C. for one hour. In the second heat treatment, part of the oxidesemiconductor layer (a channel formation region) is heated while beingin contact with the insulating layer 516.

Through the above process, the first heat treatment is performed on theoxide semiconductor film so that an impurity such as hydrogen, moisture,a hydroxyl group, or hydride (also referred to as a hydrogen compound)is intentionally removed from the oxide semiconductor layer.Additionally, oxygen which is reduced at the same time as the step forremoving impurities can be supplied. Accordingly, the oxidesemiconductor layer is highly purified to be an electrically i-type(intrinsic) semiconductor.

Through the above process, the transistor 510 is formed (FIG. 12D).

When a silicon oxide layer having a lot of defects is used as the oxideinsulating layer, heat treatment after formation of the silicon oxidelayer has an effect in diffusing an impurity such as hydrogen, moisture,a hydroxyl group, or hydride contained in the oxide semiconductor layerto the oxide insulating layer so that the impurity contained in theoxide semiconductor layer can be further reduced.

A protective insulating layer 506 may be formed over the insulatinglayer 516. For example, a silicon nitride film is formed by an RFsputtering method. Since an RF sputtering method has high productivity,it is preferably used as a deposition method of the protectiveinsulating layer. As the protective insulating layer 506, an inorganicinsulating film which does not include an impurity such as moisture andprevents entry of these from the outside, such as a silicon nitride filmor an aluminum nitride film can be used. In this embodiment, theprotective insulating layer 506 is formed using a silicon nitride film(see FIG. 12E).

In this embodiment, as the protective insulating layer 506, a siliconnitride film is formed by heating the substrate 505 over which layers upto and including the insulating layer 516 are formed, to a temperatureof 100° C. to 400° C., inclusive, introducing a sputtering gascontaining high-purity nitrogen from which hydrogen and moisture areremoved, and using a target of silicon semiconductor. In this case, theprotective insulating layer 506 is preferably deposited removingresidual moisture in a treatment chamber, similarly to the insulatinglayer 516.

After the formation of the protective insulating layer 506, heattreatment may be further performed at a temperature higher than or equalto 100° C. and lower than or equal to 200° C. in the air atmosphere forgreater than or equal to 1 hour and less than or equal to 30 hours. Thisheat treatment may be performed at a fixed heating temperature.Alternatively, the following change in the heating temperature may beconducted plural times repeatedly: the heating temperature is increasedfrom a room temperature to a temperature higher than or equal to 100° C.and lower than or equal to 200° C., inclusive and then decreased to aroom temperature.

In this manner, with the use of the transistor including ahighly-purified oxide semiconductor layer manufactured using thisembodiment, the current value in an off state (an off-state currentvalue) can be further reduced. Accordingly, an electric signal such asimage data can be held for a longer period and a writing interval can beset long. Therefore, the frequency of refresh operation can be reduced,which leads to a decrease in power consumption.

In addition, since the transistor including a highly-purified oxidesemiconductor layer has high field-effect mobility, high-speed operationis possible. Accordingly, by using the transistor in a pixel portion ofa liquid crystal display device, a high-quality image can be provided.In addition, since the transistor can be separately formed in a drivercircuit portion and a pixel portion over one substrate, the number ofcomponents of the liquid crystal display device can be reduced.

This embodiment can be implemented combining with another embodiment asappropriate.

Embodiment 7

In this embodiment, a pixel structure which enables increase in theamount of reflected light and transmitted light per one pixel in asemi-transmissive liquid crystal display device will be described withreference to FIG. 14, FIGS. 15A to 15E, and FIG. 16.

FIG. 14 is a view illustrating a plan structure of a pixel described inthis embodiment. FIGS. 15A, 15B, and 15E illustrate cross-sectionalstructures of a portion along S1-S2, a portion along T1-T2, and aportion along U1-U2, respectively, denoted by dashed lines in FIG. 14.In the pixel described in this embodiment, over a substrate 800, atransparent electrode 823 and a reflective electrode 825 which are usedas pixel electrodes are stacked with an insulating layer 824 interposedtherebetween.

The transparent electrode 823 is connected to a drain electrode 857 of atransistor 851 through a contact hole 855 provided in an insulating film827, an insulating film 828, and an organic resin film 822. The drainelectrode 857 overlaps with a capacitor wiring 853 with a gateinsulating layer interposed therebetween to form a storage capacitor 871(see FIG. 15A).

A gate electrode 858 of the transistor 851 is connected to a wiring 852,and a source electrode 856 is connected to a wiring 854. The transistordescribed in any of the other embodiments can be used for the transistor851 (see FIG. 14).

The reflective electrode 825 is connected to a drain electrode 867 of atransistor 861 through a contact hole 865 provided in the insulatingfilm 827, the insulating film 828, and the organic resin film 822 (seeFIG. 15E). The drain electrode 867 overlaps with a capacitor wiring 863with a gate insulating layer interposed therebetween to form a storagecapacitor 872.

A gate electrode 868 of the transistor 861 is connected to a wiring 862,and a source electrode 866 of the transistor 861 is connected to awiring 864. The transistor described in any of the other embodiments canbe used as the transistor 861 (see FIG. 14).

External light is reflected by the reflective electrode 825, so that thepixel electrode can function as a pixel electrode of a reflective liquidcrystal display device. The reflective electrode 825 is provided with aplurality of openings 826. In the opening 826, the reflective electrode825 does not exist, and a structure 820 and the transparent electrode823 are projected (see FIG. 15B). Light from the backlight istransmitted through the opening 826, so that the pixel electrode canfunction as a pixel electrode of a transmissive liquid crystal displaydevice.

In the semi-transmissive liquid crystal display device described in thisembodiment, the reflective electrode 825 and the transparent electrode823 are electrically isolated using the insulating layer 824. Thepotential applied to the transparent electrode 823 can be controlled bythe transistor 851, and the potential applied to the reflectiveelectrode 825 can be controlled by the transistor 861; therefore, thepotential of the reflective electrode 825 and the potential of thetransparent electrode 823 can be controlled independently. Accordingly,when the semi-transmissive liquid crystal display device functions as atransmissive type, the liquid crystal display on the reflectiveelectrode 825 can be black display.

FIG. 16 is a cross-sectional view illustrating an example different fromthat in FIG. 15B, which is one embodiment of the present inventionhaving a structure in which the structure 820 and the transparentelectrode 823 are not projected in the opening 826. In FIG. 15B, abacklight exit 841 and the opening 826 have almost the same size,whereas in FIG. 16, the backlight exit 841 and the opening 826 havedifferent sizes and different distances from a backlight entrance 842.Accordingly, the amount of transmitted light can be made larger in FIG.15B than in FIG. 16, and it can be said that the cross-sectional shapein FIG. 15B is preferable.

In a lower layer of the opening 826, the structure 820 is formed tooverlap with the opening 826. FIG. 15B is a cross-sectional view of theportion along T1-T2 in FIG. 14, which illustrates the structures of thepixel electrode and the structure 820. FIG. 15C is an enlarged view of aportion 880, and FIG. 15D is an enlarged view of a portion 881.

Reflected light 832 is external light reflected by the reflectiveelectrode 825. The top surface of the organic resin film 822 is acurving surface with an uneven shape. By reflecting the curving surfacewith an uneven shape on the reflective electrode 825, the area of thereflective region can be increased, and reflection of an object otherthan the displayed image is reduced so that visibility of the displayedimage can be improved. In the cross-sectional shape, the angle θR at apoint where the reflective electrode 825 having a curving surface ismost curved, formed by two inclined planes facing each other may begreater than or equal to 90°, preferably greater than or equal to 1000and less than or equal to 120° (see FIG. 15D).

The structure 820 includes the backlight exit 841 on the opening 826side and the backlight entrance 842 on a backlight (not illustrated)side. The upper portion of the structure 820 is positioned above thesurface of the reflective electrode 825 and protrudes from the upper endportion of the reflective electrode; that is, the distance H between theupper end portion of the structure 820 and the upper end portion of thereflective electrode is greater than or equal to 0.1 μm and less than orequal to 3 μm, preferably greater than or equal to 0.3 μm and less thanor equal to 2 m. The backlight entrance 842 is formed to have a largerarea than that of the backlight exit 841. A reflective layer 821 isformed on the side surfaces of the structure 820 (surfaces on which thebacklight exit 841 and the backlight entrance 842 are not formed). Thestructure 820 can be formed using a material having a light-transmittingproperty such as silicon oxide, silicon nitride, or silicon oxynitride.The reflective layer 821 can be formed using a material with high lightreflectance such as aluminum (Al) or silver (Ag).

Transmitted light 831 emitted from the backlight enters the structure820 through the backlight entrance 842. Some of the incident transmittedlight 831 is directly emitted from the backlight exit 841, some isreflected toward the backlight exit 841 by the reflective layer 821, andsome is further reflected to return to the backlight entrance 842.

At this time, according to the shape of a cross section of the structure820 including a line passing through the backlight exit 841 and thebacklight entrance 842, side surfaces on right and left facing eachother are inclined surfaces. The angle θT formed by the side surfaces ismade to be less than 90°, preferably greater than or equal to 10° andless than or equal to 600, so that the transmitted light 831 incidentfrom the backlight entrance 842 can be guided efficiently to thebacklight exit 841 (see FIG. 15C).

For example, in one pixel, the area of the pixel electrode is set to100%. The area of an electrode functioning as a reflective electrode isSR, and the area of an electrode functioning as a transmissive electrode(the area of the opening 826) is ST. In that case, in a conventionalsemi-transmissive liquid crystal display device, the total area of thearea SR of the electrode functioning as the reflective electrode and thearea ST of the electrode functioning as the transmissive electrodeaccounts for 100% of the area of the pixel electrode. In thesemi-transmissive liquid crystal display device having the pixelstructure described in this embodiment, the area ST of the electrodefunctioning as a transmissive electrode corresponds to the area of thebacklight entrance 842, whereby the area ST of the opening 826 can beincreased. Since the area ST of the electrode functioning as atransmissive electrode corresponds to the area of the backlight entrance842, the amount of transmitted light can be increased without increasingthe luminance of the backlight. Therefore, the total area of the area SRof the electrode functioning as a reflective electrode and the area STof the electrode functioning as a transmissive electrode can be greaterthan or equal to 100%. In other words, the proportion of the area of thepixel electrode in appearance can be greater than or equal to 100%.

By using this embodiment, a semi-transmissive liquid crystal displaydevice with bright and high-quality display can be obtained withoutincreasing power consumption.

Embodiment 8

In this embodiment, an example of an electronic device including theliquid crystal display device described in any of the above embodimentswill be described.

FIG. 13A illustrates an electronic book reader (also referred to as ane-book reader) which can include housings 9630, a display portion 9631,operation keys 9632, a solar battery 9633, and a charge and dischargecontrol circuit 9634. The electronic book reader illustrated in FIG. 13Ahas a function of displaying various kinds of information (e.g., a stillimage, a moving image, and a text image) on the display portion, afunction of displaying a calendar, a date, the time, or the like on thedisplay portion, a function of operating or editing the informationdisplayed on the display portion, a function of controlling processingby various kinds of software (programs), and the like. Note that in FIG.13A, a structure including a battery 9635 and a DCDC converter(hereinafter abbreviated as a converter 9636) is illustrated as anexample of the charge and discharge control circuit 9634.

When a semi-transmissive liquid crystal display device is used as thedisplay portion 9631, the electronic book reader is expected to be usedin a relatively bright environment, in which case the structureillustrated in FIG. 13A is preferable because power generation by thesolar battery 9633 and charge in the battery 9635 are effectivelyperformed. Note that a structure in which the solar battery 9633 isprovided on each of a surface and a rear surface of the housing 9630 ispreferable in order to charge the battery 9635 efficiently. When alithium ion battery is used as the battery 9635, there is an advantageof downsizing or the like.

The structure and the operation of the charge and discharge controlcircuit 9634 illustrated in FIG. 13A will be described with reference toa block diagram in FIG. 13B. The solar battery 9633, the battery 9635,the converter 9636, a converter 9637, switches SW1 to SW3, and thedisplay portion 9631 are illustrated in FIG. 13B, and the battery 9635,the converter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634.

First, an example of operation in the case where power is generated bythe solar battery 9633 using external light is described. The voltage ofpower generated by the solar battery is raised or lowered by theconverter 9636 so that the power has voltage for charging the battery9635. Then, when the power from the solar battery 9633 is used for theoperation of the display portion 9631, the switch SW1 is turned on andthe voltage of the power is raised or lowered by the converter 9637 soas to be voltage needed for the display portion 9631. In addition, whendisplay on the display portion 9631 is not performed, the switch SW1 isturned off and the switch SW2 is turned on so that charge of the battery9635 may be performed.

Next, operation in the case where power is not generated by the solarbattery 9633 using external light is described. The voltage of poweraccumulated in the battery 9635 is raised or lowered by the converter9637 by turning on the switch SW3. Then, power from the battery 9635 isused for the operation of the display portion 9631.

Note that although the solar battery 9633 is described as an example ofa means for charge, charge of the battery 9635 may be performed withanother means. In addition, a combination of the solar battery 9633 andanother means for charge may be used.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

This application is based on Japanese Patent Application serial no.2009-298456 filed with Japan Patent Office on Dec. 28, 2009, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A display device comprising: a displayportion comprising: a first transistor; a transparent pixel electrodeelectrically connected to the first transistor; a second transistor; anda reflective pixel electrode electrically connected to the secondtransistor, wherein the transparent pixel electrode is configured tocontrol a transmissive light intensity of a light-emitting element,wherein the reflective pixel electrode is configured to control areflective light intensity over the reflective pixel electrode, andwherein a channel forming region of the second transistor comprises anoxide semiconductor layer having a crystal region which isc-axis-aligned perpendicularly to a surface of the oxide semiconductorlayer, and an image processing circuit comprising: a memory circuitconfigured to store image signals; and a comparison circuit configuredto compare the image signals stored in the memory circuit and to detecta difference among the image signals, wherein when the comparisoncircuit detects the difference in successive frame periods, the imageprocessing circuit is configured to output a first signal associatedwith a moving image, and wherein when the comparison circuit does notdetect the difference in successive frame periods, the image processingcircuit is configured to output a third signal associated with a stillimage and stop an output of the third signal during a predeterminedperiod.
 2. The display device according to claim 1, wherein thelight-emitting element comprises different emitting colors, and whereinthe light-emitting element is configured to sequentially turn on each ofthe different emitting colors.
 3. The display device according to claim1, wherein when the comparison circuit does not detect the difference insuccessive frame periods, the image processing circuit is configured tostop output of the first signal.
 4. The display device according toclaim 1, wherein the predetermined period is one minute or longer. 5.The display device according to claim 1, wherein a liquid crystal layeris provided over the transparent pixel electrode and the reflectivepixel electrode.
 6. A display device comprising: a display portioncomprising: a first transistor; a transparent pixel electrodeelectrically connected to the first transistor; and a second transistor;and a reflective pixel electrode electrically connected to the secondtransistor, wherein the transparent pixel electrode is configured tocontrol a transmissive light intensity of a light-emitting element,wherein the reflective pixel electrode is configured to control areflective light intensity over the reflective pixel electrode, andwherein a channel forming region of the second transistor comprises anoxide semiconductor layer having a crystal region which isc-axis-aligned perpendicularly to a surface of the oxide semiconductorlayer, and a comparison circuit configured to detect whether there isdifference between image signals in successive frame periods, whereinthe display portion is configured to receive a first signal associatedwith a moving image when there is the difference, and wherein thedisplay portion is configured to receive a third signal associated witha still image when there is no difference detected by the comparisoncircuit between the image signals in the successive frames, and to stopreceiving the third signal during a predetermined period.
 7. The displaydevice according to claim 6, wherein the light-emitting elementcomprises different emitting colors, and wherein the light-emittingelement is configured to sequentially turn on each of the differentemitting colors.
 8. The display device according to claim 6, furthercomprising an image processing circuit, wherein the image processingcircuit includes a memory circuit configured to store the image signalsand the comparison circuit configured to compare the image signalsstored in the memory circuit.
 9. The display device according to claim6, wherein the display portion is configured to stop of receivereceiving of the third signal for one minute or longer.
 10. The displaydevice according to claim 6, wherein when the comparison circuit detectsthe difference in successive frame periods, the display portion isconfigured to receive the first signal and a second signal for a blackstate to different signal lines.
 11. The display device according toclaim 6, wherein when the comparison circuit does not detect thedifference in successive frame periods, the display portion isconfigured to stop of receive receiving of the first signal and a secondsignal for a black state.
 12. The display device according to claim 6,wherein a liquid crystal layer is provided over the transparent pixelelectrode and the reflective pixel electrode.